Slow-cycling cells, therapy resistance and clinical implications in prostate cancer Project Summary/Abstract Accumulating evidence suggests the existence of slow-cycling cells (SCCs) in cancer cell cultures, xenograft tumors, and human clinical tumor specimens. Although SCCs are thought to be insensitive to anticancer therapies and contribute to later disease progression and metastasis, few studies have focused on and developed systems to study SCCs. Despite an initial favorable response to initial hormone therapy, almost all prostate cancer (PCa) patients eventually develop progressive castration-resistant prostate cancer (CRPC). Mechanisms underlying CRPC development and maintenance remain poorly understood. Our meticulous studies on PCa cell heterogeneity, combined with preliminary studies on PCa SCCs lead to hypothesis that SCCs survive androgen deprivation therapy (ADT) and repopulate CRPC. By developing innovative experimental strategies to purify out SCCs live from human PCa cell cultures and xenograft tumors, and establishing a unique transgenic mouse model of label-retaining cells (LRCs), we are poised to systematically investigate SCCs and test our hypothesis in a nave tumor microenvironment under both physiological and androgen-ablation conditions. Through a spectrum of in vitro and in vivo biological assays, we will comprehensively examine the functional properties and the clinical implications of SCCs in PCa, especially in castration resistance. By performing gene expression analysis coupled with functional assays, we will find potential therapeutic targets that could lead to elimination of this `hard-to-kill' population. Our studies will not only fill critical gap knowledge of quiescent cancer cell biology but also provide clinical insights into how we can target them for better patient outcomes. The long-term goal of this project is to identify treatment options by combining ADT and SCC-specific therapies to eventually eliminate CRPC in patients.